Going from SJ to the airport, you get in the line at the Tuasa terminal for the DIRECTO bus. Those busses have signs in the front window that say DIRECTO.
Just ask the attendants marshaling the busses, "Directo Aeropuerto???". They will tell you which bus goes directly to the airport before continuing on its way other Tuasa terminal in Alejuela Centro. To be sure, you can ask the driver the same question as you get on the bus.
I was on one DIRECTO bus going to the airport when the driver announced something that I didn't understand about the aeropuerto. I hadn't asked anything before boarding the bus which I knew was the correct bus.
At the fork in the road near the airport, he started to go toward Alejuela instead of toward the airport.
I was sitting near the driver, and was able to get his attention soon enough to let him know that this stupid gringo needed the airport.
He mumbled some grumpy admonishment to me.
I'm guessing that his earlier announcement was asking if anybody needed the airport while he was driving the Directo bus.
I thought that he was announcing that the AEROPUERTO WAS NEXT.
The airport is about 15 miles from the terminal. Most of the way is on a 4-lane, divided highway. The travel time for a bus (or for a taxi) depends on the traffic. Without an accident and without the rush hour, the travel time is about 20-30 minutes. In the morning rush hour, it can take 30 minutes to get out of the city center onto the highway.
I'm glad to give the girls the $60 I save by taking the bus. They're glad, too.